1. Field of the Invention
The present invention relates to a front suspension for a two-wheeled vehicle in which a cushioning effect of the suspension is improved.
2. Background Art
Telescopic type suspensions, or bottom link type suspensions, are conventional in two-wheeled vehicles. A telescopic type suspension is capable of expansion and contraction like a telescope, and is suitable for a two-wheeled vehicle having a relatively small caster angle (the angle between a vertical line and front forks). In two-wheeled motor vehicles of large caster angle, also known as xe2x80x9cAmericanxe2x80x9d type motorbikes, it is difficult to absorb vertical movements of a front wheel using a telescopic type suspension because the front forks extend horizontally. A bottom link type suspension is not overly influenced by the caster angle, and is therefore suitable for a two-wheeled motor vehicle having a large caster angle.
A conventional bottom link type suspension is shown in Japanese Utility Model Publication No. Sho 60-15744, entitled xe2x80x9cA Front Wheel Suspension in a Two-wheeled Vehicle.xe2x80x9d According to this suspension, as shown in FIGS. 1 and 2 of the publication, a front wheel W is suspended by parallel links (reference numerals 3, 8, 6 and F), a hydraulic damper (not shown) and a suspension coil spring 14. The suspension shown in FIG. 1 is a leading type suspension, because an axle 5 of the front wheel W is located ahead of the suspension, while the suspension shown in FIG. 2 of the publication is a trailing type suspension, because the front wheel axle 5 is located behind the suspension.
In FIG. 1 of the publication, when a brake is applied to the front wheel, the front wheel W rises relatively with respect to a vehicle body frame, and jumping results in a relative descent of the front wheel W. Thus, a large suspension stroke results. In the same figure, the angle between a leading arm 3 and a link 8 is approximately 90xc2x0. As the front wheel W descends relatively, the angle becomes larger than 90xc2x0, while with a relative ascent of the front wheel W, the angle becomes smaller than 90xc2x0. Also in the above FIG. 2, the angle between the leading arm 3 and the link 8 is approximately 90xc2x0, which angle increases and decreases above and below 90xc2x0. This angular change will be explained below with reference to FIG. 7 of the present application.
FIG. 7 of the present application is a schematic diagram of FIG. 2 of Japanese Utility Model Publication No. Sho 60-15744, showing the principle elements of a conventional bottom link type suspension. The leading arm as referred to in the publication is here denoted a front wheel supporting arm 103, which is a trailing arm. One end of the front wheel supporting arm 103 is connected to a lower end of a front fork 101 and is swingable through a first pin 102, an axle 105 of a front wheel 104 is secured to the opposite end of the front wheel supporting arm 103. A lower end of a push rod 107 is connected to an intermediate position of the front wheel supporting arm 103 through a second pin 106. An upper link 109 extends from an upper portion of the front fork 101 through a third pin 108, and an upper end of the push rod 107 is connected to an intermediate position of the upper link 107 through a fourth pin 110. An end of the upper link 109 opposite to the front fork side is connected to a lower end of a suspension spring 111. Thus, the front wheel 104 is suspended by a parallel link structure.
An axis passing through both first and second pins 102, 106 is a first axis 103A, an axis passing through both second and fourth pins 106, 110 is a second axis 107A, and the angle on the front fork 101 side between the first axis 103A and the second axis 107A is the angle "psgr", which increases and decreases with upward and downward movements of the front wheel 104. That is, when the axle 105 moves from 1 to 2, the angle "psgr" becomes large, while when the axle 105 moves from 1 to 3, the angle "psgr" becomes small. In many cases "psgr" changes in the range of 70xc2x0 to 110xc2x0 on both sides of 90xc2x0.
Taking note of a vertically upward movement of the fourth pin 110, it is seen that in the region where the angle "psgr" is smaller than 90xc2x0, the amount of upward displacement of the fourth pin increases as the angle approaches 90xc2x0, while in the region where the angle "psgr" exceeds 90xc2x0, the amount of upward displacement of the fourth pin 110 decreases as the angle "psgr" becomes larger than 90xc2x0. As a sine curve, with the angle of 90xc2x0 as its peak angle, the amount of an upward displacement of the fourth pin 110 decreases before and after the peak angle of 90xc2x0.
FIG. 8 of the present application is an illustration of the axes shown in FIG. 7. The distance from a horizontal axis Ha which passes through the first pin 102 up to the fourth pin 110 is xe2x80x9cHxe2x80x9d, and the distance from the horizontal axis Ha to the second pin 106 is xe2x80x9chxe2x80x9d. The length of the first axis 103A is xe2x80x9crxe2x80x9d and the inclination angle thereof is xe2x80x9c"psgr"2xe2x80x9d, the length of the second axis 107A is xe2x80x9cRxe2x80x9d, and the inclination angle thereof is xe2x80x9c"psgr"1xe2x80x9d, with "psgr" defined by "psgr"="psgr"1+"psgr"2. The following expressions are therefore established:
"psgr"="psgr"1+"psgr"2
R cos "psgr"1=H+h
h=r cos "psgr"2
H=R cos "psgr"1xe2x88x92r cos "psgr"2xe2x80x83xe2x80x83eqn. (4)
Hxe2x80x2=xe2x88x92R sin "psgr"1+r sin "psgr"2xe2x80x83xe2x80x83eqn. (5)
sin "psgr"2=sin("psgr"xe2x88x92"psgr"1)=sin "psgr" cos "psgr"1xe2x88x92cos "psgr" sin "psgr"1xe2x80x83xe2x80x83eqn. (6)
If "psgr"=90xc2x0, sin "psgr"=1, cos "psgr"=0 sin "psgr"2=cos "psgr"1 Hxe2x80x2=xe2x88x92R sin "psgr"1+r cos "psgr"1xe2x80x83xe2x80x83eqn. (7)
As shown in the above equation (4), the distance H becomes a function of R, r, "psgr"1 and "psgr"2.
A rate of change of H can be determined by differentiation. The result of the differentiation is as shown in the above equation (5). sin "psgr"2 is defined according to equation (6).
As noted above, the angle "psgr" has heretofore been approximately 90xc2x0, so if the angle "psgr" is assumed equal to 90xc2x0, Hxe2x80x2 can be approximated by equation (7), and it becomes a function of R, r and "psgr"1.
FIG. 9 of the present application is a graph showing R sin "psgr"1 and r cos "psgr"1 plotted with respect to "psgr"1 along the axis of abscissa, according to a conventional suspension. The angle "psgr"1 becomes ("psgr"-"psgr"2) and it is presumed that there is a change at around a half of "psgr", i.e., 45xc2x0. The sine curve and the cosine curve intersect each other at an angle of between 0xc2x0 and 90xc2x0. If this intersecting point is assumed to be M, R sin "psgr"1 is larger than r cos "psgr"1 on the 90xc2x0+side with respect to the intersecting point M, so that the above equation (7) becomes negative in sign (xe2x88x92). In the 0xc2x0-90xc2x0 side with respect to the intersecting point M, R sin "psgr"1 becomes smaller than r cos "psgr", so that the above equation (7) becomes positive in sign (+).
Thus, if the angle "psgr" is near 90xc2x0 and if Hxe2x80x2 is positive, the fourth pin 110 rises while being accelerated, while if H is negative, the fourth pin rises while being decelerated. Thus, if a peak in the amount of displacement or a point of change between acceleration and deceleration is present intermediate to the vertically moving stroke of the front wheel, a cushion feeling having a sense of incongruity results.
According to the conventional art, such incongruity sense is absorbed by a suspension spring. To this end, however, a special spring whose spring coefficient varies according to the amount of compression (for example, a spring different in diameter depending position, or a multi-stage spring) must be used as the suspension spring, thus leading to an increase in cost of the suspension spring.
It is therefore an object of the present invention to provide a front wheel suspension in which a peak is not contained in a displacement ratio of the upper end of the push rod.
The present invention achieves the above object and other advantages not contemplated by the conventional art.
A front wheel suspension according to an embodiment of the present invention is a bottom link type suspension in a two-wheeled vehicle, in which a front wheel supporting arm is connected to a front fork swingably through a first pin. An axle of a front wheel is mounted to an end portion of the front wheel supporting arm, a lower end of a push rod is connected to an intermediate position of the front wheel supporting arm through a second pin, with the push rod being extended upward. An upper end of the push rod is connected through a third pin to an upper link, which is connected to a bottom bridge swingably through a fourth pin. The upper link is connected to a lower end of a suspension spring through a fifth pin.
An axis connecting the first pin on the front fork side with the second pin mounted at an intermediate position of the front wheel supporting arm is assumed to be a first axis, and an axis connecting the second pin at an intermediate position of the front wheel supporting arm with the third pin on the upper link side is assumed to be a second axis. The smaller of the two angles between the first and second axes is assumed to be xcex8. A link is constituted so that the angle xcex8 is maintained in a range not exceeding 90xc2x0 throughout the whole region of a suspension stroke of the front wheel.
If the smaller of the angles between the first and second axes is assumed to be xcex8, and if the angle xcex8 is maintained in a range not exceeding 90xc2x0 throughout the whole region of a suspension stroke of the front wheel, the amount of a vertical displacement of the third pin can be varied in a substantially linear manner, thus permitting the use of a less expensive suspension spring.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.